1. Field of the Invention
The present invention relates to superconducting wire and methods of manufacturing the same.
2. Description of the Background Art
If a multifilament superconducting wire is formed of a superconducting material of copper oxide and metal coating the superconducting material, the wire is typically produced as follows: initially, powdery oxide is loaded into a pipe of metal such as silver to produce a monofilamentary wire. Then a plurality of such monofilamentary wires are bundled and thus inserted into another pipe of metal such as silver to obtain a multifilamentary structure serving as an original wire which is in turn for example drawn and rolled and thus formed in a tape and then heat treated to provide a superconducting wire.
While it is important that such a superconducting wire has a superconducting portion with properties, such as critical temperature and critical current, for practical use, it is also essential that the coating metal portion have properties, serving as follows:
(1) Coats an oxide superconducting material of ceramic to provide flexibility;
(2) Prevents the superconducting portion from providing performance impaired by an external factor (i.e., protects the superconducting portion against the external factor); and
(3) Provides good electrical contact.
In recent years, of the above three objects, the second object has been +focused on and coating materials have thus been improved. There are two types of disturbance for oxide superconducting wire coated with metal. One is attributed to mechanical stress of tensility, bending and the like. The other is attributed to a pinhole in a wire surface, since the pinhole disadvantageously introduces a coolant into the superconducting wire and when it is heated the coolant vaporizes and expands and consequently destroys the superconducting portion.
The above two types of disturbance are limited basically by employing a metal coating portion in the form of a thick metal tube. While this can protect the superconducting portion, it also has a disadvantage; a superconductor is an oxide. As such, when it is heat treated, between the oxide and the external atmosphere a gas (oxygen in particular) is discharged and received inevitably. As such it is essential that the gas (oxygen in particular) be discharged and received smoothly (i.e., as naturally as possible) to obtain a highly pure superconducting phase. Thus, silver, having high oxygen permeability, is used as the coating member, although if the silver portion is thick its permeability would degrade.
Furthermore, if for a similar reason oxygen is not completely discharged from the wire the gas expands, resulting in the wire having a defect.
A thin tube can be used to provide a highly pure superconducting phase and prevent the wire from swelling, although it has a reduced level of strength and tends to result in more pinholes and other contrary phenomena.
A metal tube can also be increased in strength, as follows: rather than pure silver, silver with second and third metals added thereto is used as a starting material (a coating tube, referred to as an alloy tube) and it is heat treated and thus oxidized to disperse in the coating portion as an oxide to increase its strength. This silver coating tube with a second metal added thereto, however, is disadvantageous, as follows:
An oxide superconductor hardly reacts with silver and if it is in contact with silver it smoothly transitions to a superconducting phase targeted. The oxide superconductor, however, readily reacts with an element (or component) other than silver. As such, the powdery portion varies in composition and the superconducting phase targeted can thus not be highly pure. Thus, a wire using an alloy tube provides a level of critical current lower than that using a tube formed of pure silver. Furthermore, the variation in composition depends in magnitude on the concentration of the second element of the metal tube. As such, relatively low levels of concentration are often applied.
A superconducting wire in the form of a tape is provided with a second tape member stuck thereon to provide a wire significantly increased in strength, as disclosed in U.S. Pat. No. 5,801,124, Japanese Patent Laying-Open No. 4-43510, or L. Masur et al., xe2x80x9cLONG LENGTH MANUFACTURING OF BSCCO-2223 WIRE FOR MOTOR AND CABLE APPLICATIONSxe2x80x9d, International Cryogenic Materials Conference Montreal, Quebec, Canada Jul. 12-16, 1999.
U.S. Pat. No. 5,801,124 discloses that a superconducting wire coated with silver that has undergone all heat treatments is provided with a tape member of stainless steel stuck thereon with an adhesive such as solder. Japanese Patent Laying-Open No. 4-43510 discloses a metal coated, oxide superconducting wire in the form of a tape having one side brought into contact with Ni, SUS or a similar metal in the form of a tape and thus heat treated to have a reinforcement member stuck on one side.
While the wires with a second tape member stuck thereon are advantageously increased in strength, there are still disadvantageous, as follows:
In U.S. Pat. No. 5,801,124, all heat treatments must be completed before the tape member is stuck. As such the number of process steps is increased and productivity thus drops. Furthermore, the step of sticking the second tape member can serve as disturbance to impair superconductivity. Furthermore, the adhesive used to stick the second tape member further increases the entire volume and the resultant wire cannot be compact.
In accordance with the method as disclosed in Japanese Patent Laying-Open No. 4-43510, a wire in the form of a tape has only one side with a metal tape stuck thereon. As such, all of such pinholes as aforementioned cannot be covered, although the method does not contemplate covering pinholes with a metal tape. Furthermore, the metal brought into contact does not contain silver as a main component and the composition thus tends to vary. Furthermore, Ni and SUS are oxidized at high temperature more readily than silver and they thus would alter.
The present invention contemplates a superconducting wire enhanced in superconductivity and mechanical strength, capable of preventing a coolant from entering thereinto and readily configured and prepared, and a method of preparing the same.
The present invention provides a superconducting wire including a component in the form of a tape and a metal tape. The component in the form of a tape has an oxide superconducting member and a metal coating member formed mainly of silver and coating a surface of the oxide superconducting member. The metal tape does not contain any superconducting material and it is formed mainly of silver and also contains at least one component other than silver, and it is bonded in a heat treatment to a surface of the component provided in the form of a tape.
In the superconducting wire of the present invention the metal tape mainly formed of silver can reduce variation of the oxide superconductor in composition to readily provide a highly pure superconducting phase and attain a high level of critical current. Furthermore the metal tape bonded in a heat treatment on a surface of the component provided in the form of a tape, can enhance mechanical strength.
Furthermore the metal tape bonded in a heat treatment on the surface of the component provided in the form of a tape can also cover a pinhole in the metal coating member. This can prevent a coolant from entering the superconducting wire in use and expanding the wire. Since the metal tape is bonded in a heat treatment, an adhesive or a bonding step is not required. As such, the superconducting wire can be compact and the process for preparing the same can also be simplified.
Furthermore the metal tape with a second component (a component other than silver) added thereto can readily enhance mechanical strength. Furthermore, the second component of the metal tape can be prevented from reacting with the superconducting portion, since the metal coating member is posed between the metal tape and the superconducting portion and the metal tape thus does not contact the superconducting portion directly.
It should be noted that in the present invention, the metal tape is formed of a material including silver with an oxide dispersed therein. Furthermore, in the present invention, xe2x80x9cbonding in a heat treatmentxe2x80x9d conceptually includes fusion-bonding, diffusion-bonding and the like.
In the above superconducting wire preferably the component in the form of a tape has first and second main surfaces opposite to each other and the metal tape has a first portion bonded to the first main surface in a heat treatment and a second portion bonded to the second main surface in a heat treatment.
Thus the opposite surfaces can be covered with the metal tape and the metal coating member can thus have opposite surfaces with a pinhole covered with the metal tape. This can further prevent a coolant from entering and thus expanding the superconducting wire in use.
In the above superconducting wire preferably the metal tape has the first and second portions formed of different materials.
Using the metal tape sandwiching the component in the form of a tape that is formed of different materials, can provide a wire for a special application, such as requiring strength enhanced in one direction.
In the above superconducting wire preferably the metal tape is greater in mechanical strength than the component in the form of a tape.
Thus the metal tape can serve as a layer reinforcing the component in the form of a tape.
In the above superconducting wire preferably the metal tape is formed of silver with an oxide dispersed therein.
The metal tape with an oxide dispersed therein can enhance strength.
In the above superconducting wire preferably the oxide dispersed in the metal tape has a level of concentration in the metal tape higher than the concentration of an oxide in the metal coating member.
Thus the metal tape can have a level of strength higher than the metal coating member.
In the above superconducting wire preferably the oxide superconducting member is formed of a bismuth based superconductor.
For a superconducting wire as configured above a bismuth based superconductor is particularly suitable.
The present invention provides a method of manufacturing a superconducting wire, including the steps of: preparing a component in the form of a tape having a member at least with a superconducting phase, and a metal coating member formed mainly of silver and covering a surface of the member at least with a superconducting phase; and bring into contact with a surface of the component in the form of a tape a metal tape free of any superconducting material, formed of an alloy formed mainly of silver, and subjecting the component in the form of a tape and the metal tape to a heat treatment.
In the present method the component in the form of a tape and the metal tape are not fusion- or diffusion-bonded together until high temperature is attained in a heat treatment. As such, before they are fusion- or diffusion-bonded a pinhole in the metal coating member can introduce and discharge gas such as oxygen and in a heat treatment oxidization of a member containing a superconducting phase can be facilitated to obtain a highly pure superconducting phase. Furthermore, the pinhole allows gas such as oxygen to be discharged and thus output from the wire smoothly to prevent the gas from expanding the wire.
Furthermore, fusion- or diffusion-bonding the component in the form of a tape and the metal tape together in a heat treatment performed to provide a superconducting phase, can eliminate an extra step for bonding them together and thus enhance productivity. Furthermore, if sticking the metal tape onto the component in the form of a tape results in impaired superconductivity, a heat treatment that is subsequently performed can recover and enhance the impaired superconductivity.
Furthermore, fusion- or diffusion-bonding the metal tape can increase the final thickness of the portion coating the portion having a superconducting phase. As such the metal coating member itself is allowed to have a reduced thickness. As such, a pinhole is readily produced in the metal coating portion in the step of plastic forming or the step of heat treatment prior thereto. As such, if heat treatment is provided before the metal tape is stuck, a pinhole of the metal coating member allows gas such as oxygen to be smoothly introduced and discharged to readily achieve a highly pure superconducting phase and also facilitate discharging and externally outputting gas otherwise expanding the wire.
In the above method the heat treatment is performed with the component in the form of a tape having opposite, first and second main surfaces both in contact with the metal tape.
Thus the opposite surfaces can be covered with the metal tape and the metal coating member can thus have opposite surfaces with a pinhole covered with the metal tape. This can further prevent a coolant from entering and thus expanding the superconducting wire in use.
In the above method, the heat treatment performed with the component in the form of a tape having a surface in contact with the metal tape, corresponds to the final one of all heat treatments.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.